Conventional variable displacement high pressure pumps typically have a plurality of pumping elements, each of which comprises a pumping chamber in which a pumping plunger is reciprocated by a rotary cam, with fuel being supplied to it at a low pressure (approximately 40 psi) by a low pressure pump. Examples of such high pressure pumps can be found in, for example, U.S. Pat. Nos. 5,133,645; 5,094,216; 5,058,553; 4,777,921 and 4,502,445.
Furthermore, commonly a high pressure pump will have two to four pumping elements, depending upon the pumping capacity, and a respective solenoid valve is used to control the quantity of fuel metered into each of the pumping units. For costs and other related reasons, it is desirable to enable metering of the fuel into the pumping chambers of the plurality of pumping units to be controlled by no more than a single solenoid valve. In operation, conventional variable displacement high pressure pumps maintain the solenoid valves in a normally open position such that fuel flows into and fills the pumping chambers during the retraction stroke of the pumping plunger. When the pumping plunger starts its compression stroke, fuel spills back through the open solenoid valve until it receives a command signal to close. At that point, the fuel remaining in the pumping chamber is trapped and pressurized by the pumping plunger which causes the fuel to flow at high pressure into a common rail which is connected directly to a plurality of injectors or to an accumulator which may be sequentially connected to the engine injectors through a distribution valve. This being generally known as a variable start, constant stop of injection pump.
U.S. Pat. Nos. 5,109,822 and 5,035,221 disclose high pressure common rail fuel injection systems for diesel engines in which a pair of pumping elements are controlled by a single solenoid valve. However, both of the pumping elements of the pair that are controlled by the same solenoid valve are filled and discharged in unison. Accordingly, to enable fuel to be supplied to the common rail when that pair of pumping elements are being filled, a second pair of pumping elements are provided which are controlled by a second solenoid valve. Therefore, it is desirable to achieve a manner of controlling a plurality of pumping elements by way of a single solenoid valve which would enable the pumping elements to be supplied with fuel at different times and preferably 180.degree. out of phase from one another.
In an effort to overcome the above noted shortcomings, U.S. application Ser. No. 057,510 filed May 6, 1993 and assigned to the assignee of the subject invention, the contents of which are hereby incorporated herein by reference, discloses a variable displacement high pressure pumping system which includes a plurality of high pressure pumping elements which receive fuel from a low pressure fuel pump with each pumping unit having a rotary cam driven roller tappet, for producing pumping displacement of the pumping plunger of the pumping element which is connected to a respective pumping plunger by a separated link in a manner permitting the pumping plunger to float relative to the roller tappet during at least a portion of each pumping cycle thereby enabling the capacity of the pumping chamber to be limited to an extent that is less than the full stroke achievable by the pumping plunger. In this manner, the quantity of fuel to be pressurized and injected into the accumulation chamber does not have to be determined by a cutting off of a spilling flow of excess metered fuel during the compression stroke of the pumping plunger. Consequently, a low pressure solenoid valve can be used. In operation, the pumping plunger is caused to move downwardly due to a pressure differential so that fuel may be metered into a pumping chamber by way of a fuel supply line and when the electronic control unit determines that a prescribed quantity of fuel has been metered into the pumping chamber, a command signal is generated to permit low pressure fuel to be pumped to an underside of the pumping plunger thereby equalizing the pressure on both sides of the pumping plunger and consequently bringing such plunger to rest despite continued downward movement of a link plunger. Accordingly, it is necessary to ensure that the pumping plunger 3 is stopped at the exact position necessary to control the amount of fuel which is to be pumped into an accumulation chamber.
In an alternative embodiment, fuel is metered to the pumping chamber by a solenoid valve and when it is determined by an electronic control unit that a requisite amount of fuel has been metered into the pumping chamber, the solenoid valve will close. Therefore, when the pumping plunger is contacted by a tappet, the predetermined metered amount of fuel will be pressurized and passed to the accumulation chamber. However, in each of the several embodiments, the electronic solenoid valve is used merely for metering a predetermined amount of fuel into the pressure chamber which is subsequently pressurized and directed to the accumulation chamber. Further, any overflow or bypass flow of fuel is returned to the fuel supply and must be re-pumped by the low pressure pump to the pumping chamber with each of the pumping units acting independently of one another. Moreover, it is essential that the electronic control valve supplying fuel to the pumping chamber be timed to the operating cycle of the respective pumps, that is, it is necessary to time the operating cycle of the solenoid valve to the pumping cycle.
Accordingly, there is clearly a need for a high pressure pump for a fuel injection system where a plurality of high pressure pumping units maintain a pressure of fuel in an accumulation chamber at a predetermined optimum value. Further, there is a need for a high pressure pumping unit wherein at least two related pumping units can operate essentially 180.degree. out of phase with a single control valve being operated in accordance with a pressure sensed in the accumulation chamber rather than timing the operation cycle of the control valve to that of the pumping cycle.